1. Field of the Invention
The present invention relates to a sheet feeding device and a skew detecting method, and more particularly, to a sheet feeding device capable of feeding a plurality of sheets by separating the sheets one by one and a skew detecting method applicable to the sheet feeding device.
2. Description of the Related Art
An apparatus that processes a plurality of sheets, for example, an image reading apparatus such as an image scanner, a copier, a facsimile machine, or a character recognition device includes a sheet feeding device. The sheet feeding device separates stacked sheets one by one, and sequentially feeds the separated sheet to the image reading apparatus. Even when a number of sheets are stacked, the apparatus can process the sheets one by one because the sheets are automatically fed one by one. However, in such a sheet feeding device, if some of the sheets are stapled in the stack, the stapled sheets may rotate around a stapled portion, and are skewed with respect to a feeding direction because the sheets are fed even though the stapled sheets cannot move separately due to the stapling. As a result, corners of the sheets are folded and bent, and thus it may cause a damage on the sheets or a feed error, so-called a jam. A skew of a sheet may occur not only when stapled sheets are mistakenly placed in the stack, but also, for example, when a sheet being fed is caught on something else.
In a sheet feeding device disclosed in Japanese Patent Application Laid-open No. 2006-193287, a pick roller is provided at an end portion of a hopper on which sheets are stacked to pick the sheets stacked on the hopper to convey them into the device, so that the sheets are separated one by one by a separator roller and a brake roller included in a separating unit, and then fed into the apparatus. Moreover, the sheet feeding device includes a plurality of sheet detecting units arranged downstream of the separating unit to be aligned in parallel to one another in a sheet width direction (a direction perpendicular to a sheet feeding direction). In each of a plurality of detecting intervals between each two of the sheet detecting units, a skew angle is obtained based on a time difference between the sheet detecting units where a leading edge of the sheet passes through and positions of the sheet detecting units, and when a difference between the obtained skew angles exceeds a threshold, the sheet feeding device detects a skew of the leading edge of the sheet.
However, in the above sheet feeding device, a skew due to a rotation or a deformation of the sheet is detected in such a manner that each of the sheet detecting units detects a passage of the leading edge of the sheet by obtaining a skew angle with respect to the width direction based on the time difference between passages of the leading edge of the sheet through each of the sheet detecting units. Therefore, if the sheet is skewed after the leading edge of the sheet passes through the sheet detecting units, the sheet feeding device may fail to detect the skew.